Title: Nitrogen budget of a mixed-use catchment in Taita Hills, Kenya

Author: Victor Apondi Omondi

Supervising Institution: IHE Delft  - Institute for Water Education

Year: 2023



Reactive nitrogen imposes negative impacts on the Earth’s biosphere, hydrosphere and atmosphere when the level of emission and release is in excess of what these systems can sustain. This emission has been exacerbated by anthropogenic activities, since the onset of the industrial revolution, thus causing a cascade of environmental problems, including atmospheric pollution, soil acidification and eutrophication and loss of biodiversity. There is little known about the role of small shareholder African watersheds on the N fluxes and the overall N budget. This study presents the N budget of a typical tropical catchment, Wundanyi catchment in Taita hills, Kenya, which also serves as a water tower providing crucial ecosystem services to humans and other organisms. I used the NANI approach to estimate the N fluxes in the catchment in terms of N import as food and feed, biological N fixation, wet N deposition as well as synthetic N fertilizer and animal N manure application. The export pathway was explored via hydrological N loss in the river in which TN export was estimated. LULC effect on N concentration and export in sub-basins was also assessed in a nested catchment design approach. My results showed that the overall retention efficiency was 99% in WSC and 94% in Msau. Small shareholder agricultural systems greatly controlled the N fluxes in the catchment through inport of N in feeds and export of N in food, manure and fertilizer import and application in farms as well as cultivation of N fixing crops. The major source of N flux was net N in food and feed import, averaging at 33% of the total input with the second being manure N application (22%) while the least dominant was wet N deposition (10%). Riverine N export accounted for only 1% and 6% in WSC and Msau, respectively. Streams draining sub-basins dominated by forest, agriculture and settlement exported the highest amount of TN (447.26±189.6 kg d-1) while those draining forested sub-basins had the least (78.32±0.9 kg d- 1). The influence of settlements on N loss was potential as the amount of TN export increased with the presence of settlements in a sub-basin, especially in the informal settlement set-up that characterized the study area. The deficiency and inconsistency of secondary data needed for this study might have affected the budget estimation. In addition, other output pathways that were not explored, including denitrification and ammonia volatilization, could significantly impact the overall N loss. There is a need for future studies to focus on alternative output pathways as well as establish a reliable database in order to better understand the N budget of such small shareholder catchments.

Keywords: Nitrogen, NANI, riverine N export, Wundanyi catchment.